1. Field of the Invention
This invention relates to peptide analogs of agouti polypeptide and agouti-related polypeptide that are useful in modulating feeding behavior.
2. Related Art
Obesity is now recognized as a major health problem due in part to the association of obesity with cardiovascular disease, hypertension, and type II diabetes (Stark, Exp. Opin. Invest. Drugs, 7:859-864 [1998]). Obesity is believed to result from the interaction of several genetic and environmental factors. Several genes have recently been identified as having a role in feeding behavior. Some of these genes are leptin, carboxypeptidase, tubby, and agouti (Stark, supra).
The agouti gene was cloned in 1992 and was found to encode a 131 amino acid polypeptide (Bultman et al., Cell, 71:1195-1204). The human agouti polypeptide is commonly referred to as agouti signaling protein, or "ASP". Recent research has demonstrated that ASP binds to melanocortin-1 receptor and melanocortin-4 receptor (Stark, supra). Various attempts have been made to identify the amino acid residues of ASP that are important for binding. An ASP carboxy-terminal peptide encompassing amino acids 83-131 has been generated via expression cloning and is purportedly as active as full length ASP (Willard et al., Biochem., 34:12341-12346 [1995]). Several ASP amino acid variants have been prepared by expression cloning methods. For example, Kiefer et al. (Biochem., 37:991-997 [1998]) have prepared various Ala scan mutants, and Perry et al. (Genetics, 144:255-264 [1996]) have prepared two deletion mutants (desArg5-Phe14 and desArg64-Lys77) as well as various Arg, Ser and Asp substitution mutants.
Agouti related polypeptide (also referred to as "AGRP") is known to affect feeding behavior. Mice injected with AGRP peptides have been shown to increase their food uptake, resulting in obesity and diabetes (Stark, supra). Recent research suggests that AGRP is purportedly an antagonist of melanocortin-3 receptor and melanocortin-4 receptor (Fong et al., Biochim. Biophys. Res. Comm., 237:629-631 [1997]; Ollmann et al., Science, 278:135-138 [1997]). These melanocortin receptors have been implicated in weight regulation (Ollmann et al., supra).
The gene encoding human AGRP has been cloned and sequenced (Shutter et al., Genes Dev., 11:593-602 [1997]). The corresponding human polypeptide is 132 amino acids in length, and is about 25 percent identical to human agouti polypeptide. Human AGRP contains 11 cysteines, the majority of which are located at the carboxy terminal end of the polypeptide, and form 5 disulfide bridges (Bures et al., Biochemistry, 37:12172-12177 [1998]).
In an effort to identify the active region of AGRP polypeptide, various peptides of the full length molecule have been prepared and tested for activity. PCT patent application WO 97/43412 (published Nov. 20, 1997) describes an AGRP peptide of amino acids 79-132. Rossi et al. (Endocrinology, 139:4428-4431 [1998]) describe production of the AGRP peptide 83-132. Quillan et al. (FEBS Lett., 428:59-62 [1998]) describe production of the AGRP peptide 83-132, AGRP peptide 25-51, and AGRP peptide 54-82 using solid phase synthesis methods. Bures et al., supra, describe several AGRP peptides prepared by proteolytic digestion of full length recombinant AGRP including AGRP 102-112, 70-89, 90-92, 97-106, 105-112, 106-112, 75-91, 96-97, 75-91, 70-74, 64-67, 96-101, and 98-101.
In view of the need to better understand the biology of obesity, there is a need to identify and develop novel agonist and/or antagonist ligands of the melanocortin-3 and melanocortin-4 receptor subtypes with increased differential selectivity as compared with AGRP and ASP.
Accordingly, it is an object of this invention to provide molecules that can modulate, either positively or negatively, the biological activity of AGRP and ASP. This and other objects will be readily apparent to one of ordinary skill in the art.